The evolved packet core (EPC) is the core network of advanced mobile communications systems. The EPC allows different radio access technology (RATs) to operate in an integrated manner. These radio access technologies include first generation wireless local area networks (LANs), second generation (2G) systems, such as global system for mobile communication, or GSM, third generation systems (3G) such as the universal mobile telecommunication system (UMTS), and fourth generation systems (4G) such as long-term evolution (LTE).
A user equipment (UE) such as a cellphone may support multiple RATs and is known herein as a multi-mode UE. Only one RAT is operable at a time in the multi-mode UE. A multi-mode UE that is said to be “camped” on one RAT is utilizing only the technology of that RAT. The UE may be switched from one RAT to another, thus switching where the UE is camped. Thus, the multi-mode UE may be camped on LTE, get switched from the 4G RAT to the 3G RAT, and is thereafter camped on UMTS.
The older 2G and 3G RATs employ both circuit-switched (CS) and packet-switched (PS) technologies. The PS domain is used to transmit data while the CS domain is used for voice. The 4G RAT, however, is uniquely a packet-switched domain. Therefore, data and voice are transmitted using packets only. Voice calls are provided in LTE using voice-over Internet protocol (VoIP).
VoIP is implemented in LTE-capable UE systems now but is considered purely as an optional feature, due to its involvement with the IMS (internet protocol multimedia subsystem) capability of the network and the UE. VoIP also adds to the cost of the deployment of the required infrastructure for the network vendor and a subscription for the UE. System designers had these issues in mind when they developed a circuit switched fallback (CSFB) procedure to support multi-mode UEs.
Described in the 3GPP specification, CSFB enables the provisioning of voice and other circuit switched (CS) domain services to 4G-capable UEs that support multiple modes. CS domain services include unified display interface (UDI) video/LCS/USSD (LCS stands for location service and USSD stands for unstructured supplementary service data) in a multi-mode UE. A CSFB-enabled multi-mode UE has the capability to switch from a packet-switched (PS) network to a circuit-switched (CS) network in order to send or receive CS voice calls, for example.
There are some conditions in which CSFB-enabled UEs are unable to successfully perform CS domain services. A dropped phone call or other error condition results when the UE experiences one of these conditions.
Thus, there is a continuing need for an improved CSFB mechanism that overcomes the shortcomings of the prior art.